For drying of the pasty, pulpy material of high moisture content, or those of other similar consistency, various types of driers are used which--depending on the method of heat transfer--fall into two main groups: the contact or indirect and the convective or direct heat transfer system. Recently with the combination of these two systems the so-called contact-convective drier has been developed.
In driers of the contact (indirect) system the material to be dried receives the heat necessary for evaporation of the moisture through heat conduction from the wet surface in contact with it.
One of the most familiar contact driers is the single or multi-cylindrical drier for continuous operation. The internally steam heated hollow cylinder is slowly rotated in a frame construction, the material to be dried is carried in a thin layer onto the external surface with a train of rolls and the dried material is removed from the surface by a blade. In order to increase the drying capacity air is blown onto the cylinder mantle from the outside.
Another generally known type of contact drier is the heated walled mixing drier for intermittent operation. It has a vertical cylindrical which is drum externally heated and the material to be dried is carried into the interior and moved by a rotary scraper-mixer along the internal surface of the mantle, the heat transmitting surface being constantly cleaned by the scraper. The vapors arising during the process of drying are removed vacuum exhaustion or with cold air flow from the equipment.
In another familiar contact drier, bags are mounted on a hollow shaft arranged in a horizontal trough. The bags are internally and the trough externally heated with steam. The material to be dried is located in the space between the rotary bags and is loosened by blades mounted on the bags. In order to avoid simultaneous rotation, vertical blades extend between the bags ("Rotadisc" system).
For meat-meal production in animal-protein processing plants a heated screw drier is used. This drier has a double drum with heated rotary casing pipe in the interior provided with a deflecting spiral. The material to be dried passes forwardly through the annular space which is heated on both sides with the aid of the deflecting spiral rotating together with the casing pipe. The developed vapor passes through the holes of the narrow threads of the spiral casing into one or more vapor condensers mounted on the drier body.
Contact driers are of simple construction and safe operation; their heat utilization is favorable, but their applicability extends only to a narrow range of materials to be dried. On the other hand construction of the contact driers developed for drying of the materials of special consistency, e.g. pasty materials is complicated, and the operation is cumbersome. The constant renewal of the drying surface in the heated-wall mixing driers, the difficult cleaning of the interior and the high energy requirement of the mixing process cause serious problems. The high investment and operation costs represent additional disadvantages.
The material to be dried is in direct contact with the drying agent in the driers of the convective heat transfer.
One of the most familiar types of such systems is the continuous operation spray drier, in which the pulpy or pasty material is atomized to tiny drops and subjected to the effect of hot gas flow (possibly a flue gas). The drier has a large drying chamber. Atomization of the medium to be dried is mechanical, pneumatic or hydraulic. In case of necessity--since atomization of too dense material is energy- and cost-intensive, the material is made deliquescent with dispersing agents, which however are disadvantageous especially when cleanliness of the dried end product is essential. The dense material is carried into the atomizer with a screw feeder.
Fluidization driers are also known in which a stationary or moving charge is used (e.g. ceramic, glass beads, synthetic granulations) consisting of grains exceeding the grain size of the material to be dried and the fluidization drying of the pasty material or suspension takes place in the interspace or on the surface of the grains of the charge (drying of circulated layer). The grains can be created from the material to be dried. The dried product can be separated in cyclone.
The rotary drum drier (Roto-Louvre-type) was developed for the drying of non-fluidizable materials, in which the material passes from the input to the output by rotation of a drum. The mantle of the drum is "slotted" and the drying agent enters the drum tangentially and flows through the material.
A so-called swirl impulse drier fluidizes the material with air impulses ("Jet-stream"-type). The drying gas enters the vertical drum through the jet slots tangentially, agitating the material to be dried at high speed.
Disadvantages of convective type driers include unfavorable heat utilization, and energy-intensive spraying. The feed of the pasty material and operation of the sprayer are cumbersome; a large drying chamber volume is required and thus both the investment and operating costs are excessive. The charge used in the fluidization driers of the circulated-layer type increases the resistance to flow of the drying agent considerably. Consequently the use of a blower involves investment and operating cost-increasing factors.
Recently such driers have become known in which the contact and convective methods of heat transfer are combined in that the major part of the moisture is removed with convective drying in the first phase of the drying process, while the heat necessary for completion of the drying is transmitted by convective method. One of the most familiar types of driers operating on this principle is the so-called "Combined" drier developed with the combination of driers. Such driers include the cylinder belt driers and cylinder plate driers. The cylindrical drier mantle is of shaped-grooved construction. The pasty material dries during rotation of the cylinder, it develops a crusty layer which is removed with a shaped knife reaching into the grooves in the form of a comb. The so-obtained strips of material can be fed onto the after-drying belt or rotary disc.
The so-called "Devi"-type drier also uses the combined system and has a drying body of which is of cradle shape, duplicated, while the rotary shaft in the interior is fitted with a multi-threaded spiral. The drying agent is blown in at the lower part of the cradle. The mixer cuts up the encrusted material at a continuous rate and the material becomes fluidizable by the end of the first phase of the drying process. The further drying takes place in the fluidized condition of the material; the fluidizing agent is the drying agent admitted at the lower part of the cradle (e.g. hot air).
The above described combined (complex) apparatuses have been developed for the drying of certain materials of special consistency. They are generally much more complicated than driers of purely contact or convective type, and their investment and operating costs are also fairly high.